Nanomaterial could generate electricity in the dark

The findings could drive a revolution in the development of cells which convert radiated heat into electricity, known as thermophotovoltaic cells.

"Thermophotovoltaic cells have the potential to be much more efficient than solar cells," said Dr Sergey Kruk from the ANU Research School of Physics and Engineering. "Our metamaterial overcomes several obstacles and could help to unlock the potential of thermophotovoltaic cells."

Thermophotovoltaic cells have been predicted to be more than twice as efficient as conventional solar cells. They do not need direct sunlight to generate electricity, and instead can harvest heat from their surroundings in the form of infrared radiation.

They can also be combined with a burner to produce on-demand power or can recycle heat radiated by hot engines. The team's metamaterial, made of tiny nanoscopic structures of gold and magnesium fluoride, radiates heat in specific directions.

The geometry of the metamaterial can also be tweaked to give off radiation in specific spectral range, in contrast to standard materials that emit their heat in all directions as a broad range of infrared wavelengths. This makes the material suitable for use as an emitter paired with a thermophotovoltaic cell.

The project started when Dr Kruk predicted the new metamaterial would have these surprising properties. The ANU team then worked with scientists at the University of California Berkeley, who have unique expertise in manufacturing such materials.

"To fabricate this material the Berkeley team were operating at the cutting edge of technological possibilities," Dr Kruk said.

"The size of an individual building block of the metamaterial is so small that we could fit more than 12,000 of them on the cross-section of a human hair."

The research is published in Nature Communications. The key to the metamaterial's remarkable behaviour is its novel physical property, known as magnetic hyperbolic dispersion.

Dispersion describes the interactions of light with materials and can be visualised as a three-dimensional surface representing how electromagnetic radiation propagates in different directions. For natural materials, such as glass or crystals the dispersion surfaces have simple forms, spherical or ellipsoidal. The dispersion of the new metamaterial is drastically different and is hyperbolic in form. This arises from the material's remarkably strong interactions with the magnetic component of light.

The efficiency of thermophotovoltaic cells based on the metamaterial can be further improved if the emitter and the receiver have just a nanoscopic gap between them. In this configuration, radiative heat transfer between them can be more than ten times more efficient than between conventional materials.